1. Field of the Invention
The present invention relates to multiple speed sprocket drive systems like the kind found on many bicycles. More specifically, the invention is concerned with a composite spline including a hard insert on a drive body in such a system.
2. Description of the Prior Art
On bicycles, rear bicycle wheel hubs are fitted with splined drive bodies to support multiple drive sprockets. Back in the nineteen seventies, it was fairly common to see five drive sprockets on a drive body. These drive sprockets cooperated with two sprockets or chain rings on the crank to provide the rider with ten different available gear ratios or speeds. However, in recent years, in order to increase the number of “speeds” available to bicyclists, the number of drive sprockets on the drive bodies in rear bicycle wheel hubs has been increasing and the thickness of each sprocket has been decreasing in order to fit the additional sprockets into the limited space available. Designers of high performance bicycles strive to make them light in weight as performance is, generally speaking, inversely related to weight. As weight goes down, performance ratings generally go up. Splined drive bodies on which sprockets are mounted are often made of relatively hard material, i.e., steel. However, many high performance bicycle wheel hubs include splined drive bodies made of a relatively soft material, i.e., aluminum alloy.
A rear hub RH of a multiple speed bicycle is shown in FIG. 1 and generally includes a plurality of sprockets S which are mounted on an externally splined drive body DB. The sprockets S (FIGS. 1 and 1A) are internally splined in a complimentary way so that torque transmitted by a chain (not shown) to the sprockets S is, in turn, transmitted to and through the drive body DB and, in known fashion, to hub flanges HF and to the rear wheel associated with rear wheel hub RH. A conventional derailleur system (not shown) is used to move the chain (not shown) onto and off of the various sprockets S affording the rider the option to select and engage different drive ratios. The drive body DB has a tubular shape and it is provided with a plurality of splines DBS which extend a short distance radially outwardly from an outer surface of the drive body DB. In between the drive body splines DBS, there are corresponding grooves G. The grooves G and the splines DBS extend in an axial direction over most of the width of the drive body DB. Each sprocket S (FIG. 1A) has a mounting hole in its center and the hole is defined by sprocket hole splines SHS which compliment the drive body splines DBS. In this configuration, torque transmitted to a sprocket (by a chain or the like—not shown) is, in turn, transmitted to the drive body DB in the rear hub RH. Each drive body spline DBS has a force receiving face FRF (FIG. 1) which cooperates with a force transmitting face FTF (FIG. 1A) of the sprocket hole splines SHS whereby torque transmitted to one of the sprockets S is transmitted to the drive body DB.
The force transmitting faces FTF of the sprockets S have very small surface areas with the result that extremely high loads are transmitted to very small portions of the force receiving faces FRF of the drive body splines DBS. Drive bodies DB made of steel can generally withstand these high loads. However, in high performance hubs with steel sprockets and lightweight drive bodies made of aluminum alloy or of other relatively lighter but relatively softer material, the force transmitting faces FTF of the sprockets S can and do cause damage to the force receiving faces FRF of the drive body splines DBS. This will cause wear and, eventually, the sprocket and the drive body will no longer be rotationally locked relative to each other and this can accelerate the rate at which the wear occurs. In recent years, the thickness of the sprockets S has been decreasing in order to fit even more sprockets onto a drive body. One consequence of this trend is that even higher loads are being transmitted from the force transmitting faces FTF of the sprocket hole splines SHS to even smaller portions of the force receiving faces FRF of the drive body splines DBS.